Radiant electric heaters are known in which an element of coiled bare electric resistance wire is supported on a layer of thermal insulation material compacted in a metal support dish. Such heaters are described, for example, in GB 1 580 909, and are incorporated in glass-ceramic smooth top cookers. Although these operate satisfactorily, a perceived disadvantage is that they take a relatively long time, of the order of 20 to 30 seconds, to respond visibly to changes in temperature control settings, in particular when they are first energized in the cold state. This delay can be reduced by using a thinner wire which thus runs at a higher temperature; however the overall operating life of such elements may be reduced and the response time is still of the order of 8 to 10 seconds.
Another kind of radiant electric heater, described in EP 0 117 346, incorporates infra-red lamp heating elements having tungsten filaments in a fused silica envelope containing a halogen atmosphere. Such heaters have an almost instantaneous response, of the order of 1 second or less. However, because of the pronounced positive temperature coefficient of resistance of tungsten their cold resistance is much less than their hot resistance. Consequently there is a high surge current when they are first energized, leading to problems in conforming with electricity utility regulations on disturbance to electricity supplies. Furthermore, such heating elements are substantially more costly than bare wire elements.
One solution that has been suggested to the problem of slow response of electric resistance wire heaters is to energize the wire heating element at a higher power than its normal operating power for a short period after it is first energized and until it has reached its normal operating temperature. However, this technique also has difficulties associated with it. Thus, in one implementation (GB 2 199 706), a complex and expensive electronic control circuit is required. In addition, it is necessary to ensure that if the heater is de-energized and then re-energized while it is still warm, the period of higher-power operation is shorter than if the element is completely cold. Otherwise the element will be operated at excessive power while hot and will overheat, thereby reducing its operating life. This is particularly important in the case of heaters controlled by cyclic energy regulators, in which the energization of the heater is repeatedly interrupted to provide an adjustable average level of energization.
It is an object of this invention to provide a radiant electric heater with a relatively fast response, of the order of about 5 seconds or less, which alleviates some of these problems.